This invention relates to a novel and improved process for synthesizing a zeolite L-type material. In particular, the preparation procedure of this invention utilizes a seeding technique which substantially improves the nucleation and crystallization rates. This process reduces the time required for the manufacture of zeolite L-type material and the synthesis can be done under atmospheric pressure in much shorter times therefore avoiding the need to use autoclaves at elevated pressures. Furthermore, zeolite L prepared using the seeding technique is highly pure and free of zeolite T which is a common contaminant in conventional synthesis. In view of the increasing importance of developing energy efficient processing, such processes have major industrial importance. Compared to the conventional processes for preparing L-type zeolites, the method of this invention is superior from the viewpoint of energy utilization for a given product quality and yield.
A synthetic, crystalline potassium-containing zeolite designated as zeolite L has been known to exhibit good catalytic properties, particularly for hydrocarbon conversion, and advantageous sorbent properties as described, for example, by Barrer et al. Surface Science, 12, 341 (1968). The chemical composition of zeolite L is described in U.S. Pat. No. 3,216,789 as an aluminosilicate of the formula: EQU 0.9-1.3M.sub.2/n O:Al.sub.2 O.sub.3 :5.2-6.9SiO.sub.2 :yH.sub.2 O
where M is an exchangeable cation of valence n and y is from 0 to 9. Zeolite L has a characteristic X-ray diffraction pattern with the following more significant d(.ANG.) values:
16.1.+-.0.3 PA1 7.52.+-.0.04 PA1 6.00.+-.0.04 PA1 4.57.+-.0.04 PA1 4.35.+-.0.04 PA1 3.91.+-.0.02 PA1 3.47.+-.0.02 PA1 3.28.+-.0.02 PA1 3.17.+-.0.01 PA1 3.07.+-.0.01 PA1 2.91.+-.0.01 PA1 2.65.+-.0.01 PA1 2.46.+-.0.01 PA1 2.42.+-.0.01 PA1 2.19.+-.0.01
The preparation of zeolite L as described in U.S. Pat. No. 3,216,789 comprises crystallizing the zeolite from a reaction mixture having mole ratios: EQU K.sub.2 O/(K.sub.2 O+Na.sub.2 O)=0.33.about.1 EQU (K.sub.2 O+Na.sub.2 O)/SiO.sub.2 =0.35.about.0.5 EQU SiO.sub.2 /Al.sub.2 O.sub.3 =10.about.28 EQU H.sub.2 O/(K.sub.2 O+Na.sub.2 O)=15.about.41
A common by-product when utilizing the zeolite L synthesis procedure of U.S. Pat No. 3,216,789 is zeolite T (offretite type) contaminant, which has been described in U.S. Pat. No. 4,888,105 below.
British Pat. No. 1,202,511 describes a revised zeolite L preparation which comprises mole ratio of reactants as: EQU K.sub.2 O/(K.sub.2 O+Na.sub.2 O)=0.7.about.1 EQU (K.sub.2 O+Na.sub.2 O)/SiO.sub.2 =0.23.about.0.35 EQU SiO.sub.2 /Al.sub.2 O.sub.3 =6.7.about.9.5 EQU H.sub.2 O/(K.sub.2 O+Na.sub.2 O)=10.5.about.50
U.S. Pat. No. 3,867,512 discloses a preparation of zeolite L from a reaction mixture having a molar composition: EQU K.sub.2 O/(K.sub.2 O+Na.sub.2 O)=0.3.about.1 EQU (K.sub.2 O+Na.sub.2 O)/SiO.sub.2 =0.3.about.0.6 EQU SiO.sub.2 /Al.sub.2 O.sub.3 =10.about.40 EQU H.sub.2 O/(K.sub.2 O+Na.sub.2 O)=15.about.140
in which the silica source is a gel having at least 4.5 weight percent water and prepared in a particular manner.
L. Wilkosz in Pr Chem 409 (1974)-Chemical Abstract, vol 90 (1979) 57347f describes the preparation of zeolite L from a synthesis sol prepared by treating a solution containing silica, potassium hydroxide and sodium hydroxide with a second solution containing potassium aluminate, potassium hydroxide and sodium hydroxide and crystallizing for 72 hours at 20.degree. C. and 122 hours at 100.degree. C. The zeolite L product has a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 6.4, derived from input stoichiometries having SiO.sub.2 /Al.sub.2 O.sub.3 ratios between 15 and 30.
G. V. Tsitsishvilli et al in Doklady Akademii NaukSSSR, Vol 243, pp 438-440 (1978) describe the synthesis of zeolite L from alumina-silica gels containing tributylamine. The gel has the following molar ratios: EQU K.sub.2 O/Na.sub.2 O=0.5 EQU (K.sub.2 O+Na.sub.2 O)/SiO.sub.2 =0.72 EQU SiO.sub.2 /Al.sub.2 O.sub.3 =25 EQU H.sub.2 O/ (K.sub.2 O+Na.sub.2 O)=20 EQU (K.sub.2 O+Na.sub.2 O)/Al.sub.2 O.sub.3 =18
Y. Nishiimura in Nippon Kagaku Zasshi 91,11,1970, pp 1046-9 describes in general terms zeolite L preparation from a synthesis mixture containing colloidal silica, potassium aluminate and potassium hydroxide having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 15.about.25, but exemplifies only two synthesis mixtures having the following ratios of components: EQU 7K.sub.2 O:Al.sub.2 O.sub.3 :20SiO.sub.2 :450H.sub.2 O; and EQU 8K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :500H.sub.2 O.
EP 0096479 describes a synthesis procedure of zeolite L having the following composition expressed as molar ratios: EQU 2.60K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :160H.sub.2 O
Scanning electron micrograph (SEM) shows the formed zeolite L to have a cylindrical morphology.
Other workers have prepared zeolite L from gels, mixed base systems and metakaolin. See, e.g., Aiello and Barrer, J. Chem. Soc. Dalton, 1470(1970); Barter et al., J. Chem, Soc. Dalton, 1259 (1972); Barrer et al., J. Chem. Soc. Dalton, 934 (1974); U.S. Pat. No. 3,867,512 to Young and U.S. Pat. No. 4,956,166 to Verduijn.
In the synthesis of crystalline materials, addition of seeds may sometimes facilitate the nucleation and crystal growth of the desired crystalline products. Seeding techniques have been applied to the synthesis of zeolitic materials, which may substantially reduce the time of zeolite formation. The seeds may be microcrystalline materials or XRD-amorphous materials. The following are a summary of U.S. patents in which XRD-amorphous seeds have been used in the synthesis of faujasite zeolite and preformed zeolite L seeds have been used in the synthesis of zeolite L. As it will be seen in the Description of the Preferred Embodiments and Examples below, the present invention is drastically different from previous seeding techniques in that it uses an XRD-amorphous seeding gel, which does not contain potassium, in the synthesis of zeolite L.
In U.S. Pat. No. 3,639,099 Elliot, Jr. discusses the seeding technique as applied to the production of faujasite.
In U.S. Pat. No. 3,808,326 and U.S. Pat. No. 4,166,099 McDaniel et al. describe a method for preparing crystalline aluminosilicate zeolites which involves seeding an alkaline precursor mixture of alumina and silica with small size zeolite seeds, and subsequently reacting the mixture to obtain substantial yields of crystalline zeolite Y. Specifically, they have found that the processing time required to obtain substantial yields of crystalline faujasite zeolite may be drastically reduced by adding as little as from about 0.1 percent to about 10 percent by weight of an extremely finely divided zeolite seeds having particle size below about 0.1 micron to a precursor mixture of alkali metal hydroxide, alumina, and silica. These zeolite seeding gels are prepared by mixing sodium aluminate solution with sodium silicate solution at 0.degree. C. and stirring for one hour at 0.degree. C., and aging at 20.degree. C. for 4 to 16 hours without stirring. The composition of zeolite seeds are said to be of the general formula assigned to synthetic faujasite. The difference between these two patents and the present invention is that these two patents describe the synthesis of faujasite zeolite, not L zeolite.
In U.S. Pat. No. 4,343,723 Rogers et al. discuss the general concept of facilitating crystalline aluminosilicate formation by the use of seeding techniques which substantially reduce reaction time. The seeds are prepared by reacting silica-alumina alkali metal hydroxide and water in appropriate ratios under conditions of time and temperature which yield finely divided silica-alumina seed particles having an average particle size of less than about 0.1 micron. The seeds are then admixed with other reactants to form a mixture which is heated for 18 hours at 205.degree. F. to form 36% faujasite and another unidentified crystalline product. The difference between this patent and the present invention is that this patent describes the synthesis of faujasite zeolite, not L zeolite.
U.S. Pat. No. 4,657,749 describes an approximately stoichiometric synthesis of zeolite L at temperatures from between 80.degree. to 260.degree. C. from gel mixtures which have composition within the following effective general ranges: EQU K.sub.2 O:Al.sub.2 O.sub.3 =1.0.about.3.3 EQU Na.sub.2 O: Al.sub.2 O.sub.3 =0.about.1.0 EQU SiO.sub.2 :Al.sub.2 O.sub.3 =5.about.12 EQU H.sub.2 O:Al.sub.2 O.sub.3 =80.about.140
The shortening of nucleation time is also discussed in the patent for zeolite L synthesis by adding preformed zeolite L as seeds and by aging the reaction slurry or a portion of the slurry at about 10.degree. to 100.degree. C. for about 6 hours to 6 days prior to the crystallization at 80.degree.-160.degree. C. No examples are given which utilize seeding or aging techniques.
U.S. Pat. No. 4,701,315 discloses a synthesis procedure for zeolite L at 150.degree. C. for 72 hours from a crystallizing gel comprising in molar ratios of oxides: EQU K.sub.2 O/SiO.sub.2 =0.22.about.0.36 EQU H.sub.2 O/K.sub.2 O=25.about.90 EQU SiO.sub.2 /Al.sub.2 O.sub.3 =6.about.15
Although the introduction of seeds of preformed zeolite L is discussed in the patent to increase the nucleation rate in the synthesis of zeolite L, it does not present any examples in which seeding techniques have been applied in the preparation of zeolite L.
U.S. Pat. No. 4,701,315 also gives comparative examples which include a reaction at 100.degree. C. and thus may appear to be more relevant to the present invention. In Comparative Example B the synthesis procedure described in Y. Nishimura, Nippon Kagaku Zasshi 91, 11, 1970, pp 1046-9 is repeated to prepare a gel composition EQU 8.0K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :500H.sub.2 O
which is crystallized at 100.degree. C. for 65 hours. The product of this Comparative Example is essentially zeolite W, and no zeolite L is formed.
U.S. Pat. No. 4,888,105 describes the use of crystals of zeolite L as seeds and use of tetraalkylammonium cations to suppress the by-product formation of zeolite T (offretite) in the preparation of zeolite L from a reaction mixture having a molar ratio of oxides as follows: EQU SiO.sub.2 /Al.sub.2 O.sub.3 =6 to 30 EQU OH.sup.- /SiO.sub.2 =0.3 to 0.8 EQU K.sup.+ /(K.sup.+ +Na.sup.+)=0.3 to 1 EQU RN/Al.sub.2 O.sub.3 =0.2.about.2 EQU H.sub.2 O/OH.sup.- =10 to 50 EQU seeds=0 to 10%
wherein seeds are crystals of zeolite L, and RN is a source of tetraalkylammonium cation. In the sole example of preparation of this patent, however, no seeds are added to the reaction mixture during the whole synthesis process of zeolite L.
Although there are references in some patents to making zeolite L with seeds of only zeolite L, as the analysis above shows none of the patents actually included an example where they used the zeolite L seeding gel. Furthermore, the present invention uses a seeding gel which does not contain any preformed crystalline zeolite L, which is drastically different from the prior art. Before the present invention, adding a non-zeolite L seeding gel to shorten the synthesis of zeolite L was never discussed.
It is therefore an object of this invention to use seeding techniques to facilitate the nucleation and crystallization of a zeolitic aluminosilicate product.
It is a further object of this invention to use seeding techniques to facilitate the nucleation and crystallization of zeolite L.
It is a further object of this invention to use seeding techniques to facilitate zeolite L formation at 100.degree. C. and ambient pressure.
These and further objects of the present invention will become readily apparent to one skilled in the art as the description of the invention proceeds.